In recent years, mobile phones have been increasingly reduced in size and weight and accordingly, reduction in size and weight has also been required for antennas mounted on mobile phones. In the mobile phone, a dielectric antenna, the body part of which is formed with a dielectric block, is usually used, and for keeping up with the reduction in size and weight described above, a dielectric material used for the dielectric block is desired to have a high dielectric constant and a low specific gravity.
On the other hand, antenna built-in mobile phones are mainstream from the viewpoint of designability of mobile phones. The antenna built-in type also contributes to downsizing of mobile phones. Consequently, a dielectric antenna is required which has such excellent processability that the shape of its dielectric block can be flexibly changed so that the dielectric antenna can be placed in a narrow space such as a gap between a main frame of a mobile phone and a wiring board within the main frame.
Under such a background, a composite material of a filler including a dielectric ceramic and a resin is favorably used as a material of the dielectric block. This is because if such a composite material is used, the dielectric ceramic increases the dielectric constant of the dielectric block, and resultantly allows downsizing of the dielectric antenna, and the resin contributes to reduction in weight and allows the dielectric block to be formed into any form with freedom.
A composite material, which is as described above and also interesting to this invention, is described in, for example, Japanese Patent Application Laid-Open No. 2004-6316 (Patent Document 1). Patent Document 1 describes a composite dielectric material including a dielectric inorganic filler and an organic polymer material, and discloses the use of polypropylene as an organic polymer material.
However, the composite material described in Patent Document 1 does not have excellent heat resistance because polypropylene is used as an organic polymer material, and can encounter such a problem that a solder reflow process cannot be applied for mounting a dielectric antenna including a dielectric block formed with the composite material.
Thus, it has been proposed to use, as a resin, a liquid crystal polymer having excellent heat resistance in a composite material to form a dielectric block. However, the use of a liquid crystal polymer may encounter such a problem that stringing occurs during injection molding of a dielectric block, and consequently continuous molding is hindered.
For solving the problem of stringing described above while solving the problem of heat resistance, it is proposed in, for example, Japanese Patent Application Laid-Open No. 2009-197209 (Patent Document 2) to use in a composite material two different kinds of liquid crystal polymers, of which the gradients from an endothermic peak temperature at which the polymer is phase-transferred from a solid phase to a liquid phase to a temperature at which the polymer has a melt viscosity satisfying 1000 Pa·s or less are mutually different.
However, it has been found that there arises such a problem that a part of a molding material is deposited and remains on a mold when injection molding of a dielectric block is continuously carried out using the material described in Patent Document 2. This hinders continuous molding of dielectric blocks to reduce yields in production of dielectric antennas, resulting in an increase in costs of dielectric antennas.
Patent Document 1: Japanese Patent Application Laid-Open No. 2004-6316
Patent Document 2: Japanese Patent Application Laid-Open No. 2009-197209